Author

Robertsson, Johan Olof Anders

Date

1995

Advisor

Levander, Alan R.

Degree

Doctor of Philosophy

Abstract

The highly heterogeneous and anelastic nature of deep ocean seafloor results in complex reverberation as acoustic energy incident from the overlaying water column interacts and scatters from it. To gain a deeper understanding of the mechanisms causing the reverberation in sonar and seafloor scattering experiments, we have developed numerical simulation techniques that are capable of modeling the principal physical properties of complex seafloor structures. A new viscoelastic finite-difference technique for modeling anelastic wave propagation in 2-D and 3-D heterogeneous media, as well as a computationally optimally efficient method for quantifying the anelastic properties in terms of viscoelastic mechanics are presented. A method for reducing numerical dispersion using a Galerkin-wavelet formulation that enables large computational savings is also presented. The widely different regimes of wave propagation occurring in ocean acoustic problems motivate the use of hybrid simulation techniques. HARVEST (Hybrid Adaptive Regime Visco-Elastic Simulation Technique) combines solutions from Gaussian beams, viscoelastic finite-differences, and Kirchhoff extrapolation, to simulate large offset scattering problems.
Several scattering hypotheses based on finite-difference simulations of short-range acoustic scattering from realistic seafloor models are presented. Anelastic sediments on the seafloor are found to have a significant impact on the backscattered field from low grazing angle scattering experiments. In addition, small perturbations in the sediment compressional velocity can also dramatically alter the backscattered field due to transitions between pre- and post-critical reflection regimes.
The hybrid techniques are employed to simulate deep ocean acoustic reverberation data collected in the vicinity of the northern mid-Atlantic ridge. In general, the simulated data compare well to the real data. Noise partly due to side-lobes in the beam-pattern of the receiver-array is the principal source of reverberation at lower levels. Overall, the employed seafloor models were found to model the real seafloor well. Inaccurately predicted events may partly be attributed to the intrinsic uncertainty in the stochastic seafloor models. For optimal comparison between real and HARVEST simulated data the experimental geometry should be chosen so that 3-D effects may be ignored, and to yield a cross-range resolution in the beam-formed acoustic data that is small relative to the lineation of the seafloor.